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failed blower resistor (final stage resistor) not really dead

213041 Views 182 Replies 55 Participants Last post by  jabol240
This a well know trouble with the A/C computer talking to a deaf unit, the final stage resistor.
Today i found something that you may find interesting, at least for those who have this issue, fan not working anymore or working for a moment and then stop or with a fan that is not that efficient as it should be.

My final stage resistor was allowing only 10 minutes of fan with cold car and btw cold resistor.
So the usual path is remove and replace the resistor with a new one...but curiosity didnt't killed the cat and i decided to have a look inside this piece of aluminium. First trying to identify or define of what is made the material covering all the electric things within the resistor. By experience i know it won't be removable with some solvents, so i thought for a post mortem studying, thermal gun (hot air gun) would be good enough to remove the plastic thing. moulded in the aluminium part. Started to heat the plastic but without letting temperature to raise enough to not melt the plastic. This help to make the plastic a bit more soft, i used blade to remove plastic carefully to not damage some electric parts under plastic, at this moment i had no idea of what is under the cover of plastic.

With patience i removed plastic bit after bit and a small part of circuit appeared. Bad luck i guess, the circuit is built with cms components. I tried to connect the final stage resistor back to it's circuit....and nothing.
I lost my time, i was on the way to remove it when i touched the capacity (the brown part in the middle of the circuit) with finger and in the same time the aluminium body of the final stage resistor.
The fan started blowing at 100%, so i tried to change speed from the A/C control but nothing was changing.
I removed the fsr with the idea to clean a bit more the circuit and refresh soldering.
I started cleaning more surface of the circuit and i remembered one thing from my Porsche experience, the 928 was known to have terminal ECU failures due to resin which retracts while getting old with the result of breaking hybrid circuits and removing soldering from some cms components from mainboard.
Bmw seems to have same issues, later i plugged back the fsr, tried and bingo.
I filled the space where was the plastic with some silicon paste designed to be used with heatsinks. The paste i used is the white one, never use silicon with metal particles in it, it would fry everything still valid in the fsr.
Silicon won't harden like resins and it will protect from dust. This solution is not the best but it's probably the most neutral at the mechanical point of view compared to resin.

Now my fan works as it never worked since i bought my car...fsr was probably faulty from the beginning but i wasn't in condition to notice it as i always thought that this smooth working fan was normal.

So it worked for me, if you heat gently the fsr without attempting to soften too much the plastic and use some patience with the blade to remove plastic without scratching the components or the circuit then you will probably get a good surprise.
If it doesn't work please don't complain i can't predict if it would fix your fsr but if you believe it is dead you can give a try. :)

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my fsu was replaced only twice

once by dealer within warranty period with old style FSU, A few years later outside of warranty it failed. I replaced it with newer version, more fins, no issues since . my car a 2000 528 is 13 years old. I know I just jinksed my self,
The filler is a papery gray substance, black only on top. Do you think I can melt it out more gracefully than chiseling?
(I'm worried about damaging the components on the board, as some have done above.)

I wasn't sure whether to start a new short thread, or to post here, but, I've seen enough butchered keys and plastic flakes in ABS control module goop to ask before attempting to clean out my FSU.

There are plenty of pictures of cleaned out FSUs (showing the circuit board), and even some (see above) where they broke components off the circuit board, but no pictures (to my knowledge) of HOW they removed the black goop at the bottom of the FSU.

So, before I ruin mine, may I ask: How do you suggest we clean this stuff out without damaging the circuit board underneath. (I really wish I knew what I was doing.)

How deep is that circuit board anyway?
The fill looks VERY shallow in some of the pictures - but my chisel went in deeply as shown below.

EDIT: What 'is' this gray papery stuff anyway? It seems to have filaments throughout it, like heavy cardboard or something?

On this third pass, I hit some of the components!
There must be a more graceful way to remove this papery stuff.

OK. I'm about to ruin this thing as more components are popping up!
I tried to see if I could melt it out, but the stuff just burns!

Well, the stuff is rubbery like a pencil eraser, and about 1/8" inch thick. I wish there was a more graceful way, but here is where I am:

Digging onward ...

Unfortunately, at this point, I've already ruined the FSU (but I don't know it yet), as that shiny thing below is the INSIDE of a chip!

It's getting dangerously close to the components ...

At some point, the chisel was far too large and impassionate, so I tried these dental picks, which were too small & personal:

Now it's getting nearer to the components, which I don't want to knock off the board (or cut a trace):

Even though it's ruined, I may as well continue to dig away, just to see what's down there:


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Wow, it's great that this thread is still going. I visited a couple of years ago to find out about my blown FSU. It's nice to see progress from when I posted my photos of a resoldering fix.

The reason I'm back is that my fixed FSU has failed again as expected. However 2 years ain't too bad for an experimental fix!

Anyway, I am choosing to buy a new FSU and I am only considering brand names whether from eBay or not. From eBay I'm looking at 3 brands so far: Hella/Behr, Erst Wahle, Behr. Except for the Erst Wahle, they state in their product descriptions that they are German made and to beware of chinese copies etc. Prices are around $70 delivered.

According to Bluebee's in-depth analysis and polling of BMW dealerships, it seems the brand to go for is Valeo and possibly Sitronic, as that is what BMW supplies.

Can anyone confirm this? And a source for purchasing from would be great too!
Congrats as you have become Bee's here for life! Great find. Couple points maybe made elsewhere, but did not feel like reading every post, sorry. FSU would be at its COOLEST when fan is on high- think about it. FSU is absorbing or resisting power to the fan the least. FSU would be hottest when on lowest speed-BTW, my "auto" function never goes below three lights. That seems to be "autos" lowest setting. Manually, I can go to one or two lights for fan speed. Do not recommend using thermal conductive white paste to fill void. I would use a non-conductive silicon or just wrap entire FSU with electrical tape and leave it open. good going all and for Bee pushing for the answer as I would never have thought it repairable-it really isn't with surface mount components but never thought of just giving components room to expand. Would feel better if we could identify parts to resolder. Maybe someone touches on this. Will read all later.

Blue's life is in order again!! Can start her canonizing :) :bang:
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cn90 posted some useful information here about what some of the FSU pins actually do and how to test 'em ...

Also kraft apparently FIXED his FSU by gouging out the resin, resoldering connections, and then filling the holes with silicone!

If I understand correctly, Kraft did NOT use silicone, but instead used the white thermal conductive paste that is normally found between a heat sink and the thin mica wafer that isolates say a transistor from the heatsink so that the transistor can transfer as much heat as possible to the heatsink, yet not be electrically conductive between the case of the transistor (which is an active part of the transistor-could be base, emitter, collector) and the heatsink. A metal transistor, for example a power transistor, generally does not use the case as an inactive part of the transistor-especially not a ground) I would just leave the shaved away area "open". Who knows, you may have to go back there again. Maybe it is the heat from the hair drier that is somehow fixing the problem or the pressing of the knife as you shave away the black potting material. I am just a tad uncomfortable that we did not solder anything in some cases and we hope the cure is long-lasting. I would feel better if we could build upon kraft's great discovery and figure out exactly what is coming loose or being crushed by plastic? I like what I think cnn did (figures it would be cnn!) and remove all of the plastic potting material as possible so that using a non-conductive pointed coil adjusting tool probe (not a pencil!) with a used or SIM fan connected to FSU and see exactly where you touch FSU to get it to work again...try to pinpoint offending area. Why have any potting material put back at all? Just tape well to protect and let it breathe!
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As an aside, I tried to find more information about the ELMOS 0901D chip (aka E109.01D), but it's very hard to find a datasheet on it.

Here's are sources of the chip:

Here is a guy who tried to find the datasheet on the chip:

Translation: The controller turns the stove Citroen Evasion is mikruha Elmos 10901D. Broke brain search, help me find datasheet.
Translation: lobezik, in my opinion must be sought here: At least this is just their product.
Translation: lobezik, decorated the topic so that the meaning was clear from the title. For the first time fix for you.
Translation: lobezik, you never had to use tech support? I thus find analogies opamp from AD, changing product Texas Instruments ... Write a letter to them requesting datasheet. The firm is a miserable and fresh client not throw. A mikruha still produce them!
Translation: I am sorry, I apologize. ADDED 16/12/2011 14:46
Translation: Here tree sticks, you can just write to tech support, I couple blunted. Thank you!
Translation: Received a response from tehpodderzhi Elmos, cultural refused.
Thank you very much for your interest in our products.
Unfortunately we are strictly not allowed to provide any information concerning the E109.01D to you as this product is customer specific.
Best regards,
Sina Staufenberg
staatl. geprüfte Betriebswirtin (Recht)

Translation: Here at kitaezov mentioned 10901S, but datasheets do not give.
But write this type of comparator LM711.
And here the LM711, I do not know. Do it and the body 14 pins ... frustration, disappointment
Try to look for installation, can they really pushed back comparators.


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the 2 bridges are in fact 2 resistors 10 milliohm used to balance the currents between the 2 MOSFET
Does anyone have a part number on these two MOSFET transistors?

They appear to be the CRITICAL component in the FSU, as shown in the wiring diagram.

Yet, I can't find a single picture of these two MOSFETs.


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The semi-conductors circled in Blues BMW diagram are not the MOSFET you are looking for information on. The Mosfet is 14 pin and what Blue has circled is something different. I think if these Mosfets were in 1000 BMW owner's hands, maybe one person out of 1000, if that, would have the skills to install in the carcass we have left after removing all the potting material. How are we determining the MOSFET is bad in any situation? Again, even with a data sheet which we will never get because these Mosfets are supposedly "special" to BMW (just like they try to hide their MAF sensor). Few would have the skills to test a 14 pin Mosfet. I believe the FSUs that have been "repaired" are again the usual "open" circuit BMW seems to like so much (can we say ABS module).

The usual mode that OEMs use to make their parts source-able only through the OEM is by taking a standard part- say a standard motor made by GE- and GE will make some standard motors (easily obtainable by anyone with a GE part number on it) and set them aside to be stamped with an OEMs part number instead of the GE part number anyone could obtain through any GE distributor. Sometimes you might source a replacement just by taking a chance and buying a motor with same frame#, same voltage, same RPM, etc., but you are never sure the motor will work like the one supplied by the OEM. I am surprised that the part number stamped on this Mosfet can be found for sale anywhere. This is not to say the same Mosfet is not out there for sale for a quarter anywhere, but the part number should not be found listed anywhere. Makes me think it is not really re-stamped with a BMW part#, but possibly the "D" added to denote sold to BMW or the 01D just added to a std. mosfet. Much ado about nothing, sorry. rant off.
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This was posted elsewhere on the FSU operation:
I think the IC is a dual, high side motor driver IC and each transistor has a large emitter resistor with different values. That is the metal strips on the pins by the sets of three holes. That would give three speeds & off. Like a three way lamp with 50 + 100 Watt elements. Turn both on and it draws 150 Watts. This would explain the failure of the solder joints on the transistors since they are used in linear mode.

00 would be off
01 would be low speed
10 would be medium speed
11 would be high speed
Does anyone have a part number on these two MOSFET transistors?

They appear to be the CRITICAL component in the FSU, as shown in the wiring diagram.

Yet, I can't find a single picture of these two MOSFETs.
There is only ONE mosfet in the FSU and I would forget about trying to replace it even if you had one. Also, we have no reason to believe the mosfet has caused any FSUs to fail. I have yet to see the resistors in any of the picture shown anywhere in this thread. Can someone circle the resistors for me? I would think they would be fairly decent wattage so they would be very easy to see, but I don't see any resistors.
There is only ONE mosfet in the FSU and I would forget about trying to replace it even if you had one.
I'm not sure if there are one or two, because I broke one in half, and I don't see the other one (yet).

It's an epoxy resin filled with silica. It will come off with DMSO at three atmospheres or so. Sometimes a soak in DMSO for a couple weeks will make it peel off. This will also soften the PC board though.
Without soaking for days in DMSO, there's just no graceful way to get that epoxy off that transistor (cut in half by accident, in my photo below).

There is only ONE mosfet in the FSU
How did you know this? It "looks" like there are two sets of three closely spaced holes - so how did you know there was only 1 transistor?
Note: I think it 'could' be that there are actually two; but I haven't 'found' the second one, if it exists.

Elsewhere, it was said:
the spacing of the three holes is consistent with a TO-220 transistor pair, and if it's a linear pass regulator like it appears to be, there needs to be a big transistor somewhere. Also, of course, there is the heatsink.

That IC is only control logic, it just takes some mystery input signal and produces a variable voltage for the transistor base. Those two transistors are doing all the hard work. BUT, if you want to replace the device with a retrofit one, you need to know what that mysterious input signal is.


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Still would like someone to point out these two resistors we keep talking about. And I do not understand what the guy meant by some comment about "a bridge" actually being the resistor. What is he really saying as there is no electronic definition of a bridge. Bridge rectifier sure, but is he saying a resistor was replaced by a length of wire?? That makes no sense. Someone just show me these resistors in one of the pictures here-please circle.
Someone just show me these resistors in one of the pictures here-please circle.
I think someone said earlier in this thread that these "bridges" are the resistors:
- the 2 bridges are in fact 2 resistors 10 milliohm used to balance the currents between the 2 MOSFET and balance the power also. The mesure of the DC voltage on those resistors can be used to evaluate the current of the blower and its worn state.
jim_beam said:
with two resistors in series, the voltage across any one is a function hof its resistance and the current in the current in the other.
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Ok, so these resistors are a hunk of some metal (I assume metal) that has highly resistive properties and will withstand the high current that wants to pass through them. If you look closely, the lower resistive material is tapped about 1/3 of the length of the resistor material. This material must increase in resistance based upon the length of the material. The lower resistor being the same length so that it can be properly supported at both ends and such that only one length of resistive material needs to be supplied. The lower resistor just is tapped part way down the length so it has effectively a lower resistance than the top one...interesting.
Oh, please excuse me for not re-reading all posts. Blue may need to double check if my comments are pertinent. The two resistors or "bridges" as some have named can not be in the mega-ohm range as I *think* someone may have reported. If I got this wrong, well...never mind!

Keep in mind E=I*R We know E=12VDC We can infer current or *I* is probably in the range of 12-maybe 18 amps as circuit is fused at what, 40 amps? Therefore, the resistance of this piece of metal, one of which is less resistance because it is tapped, should be in range of about 1 lonely ohm to about 2 ohms max. to pass the kind of current necessary. Be sure, when testing lower resistor not to check widest two mounting points for resistance only, but to test from each "end mounting post" to the center-tap
(point 3/4s of the way down resistor where the resistor is tapped into to obtain a lessor resistance than the top resistor) Check where it is tapped to both mounting end points. Metal could be like Tungsten which we know acts differently from the metal used in say an incandescent lamp. It is likely not tungsten, but a similar metal known to have a predictable resistance depending on the length.
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The other thing that I want to know is how the circuit works?

I measure the volts in the connector:

1) + battery
2) - Ground
3) + fan
4) - fan
5) 2-8 V according to velocity roulette selectión

I undestand that the roulette potenciometer send the variable voltage to the microcontroler that makes an A/D conversión and consecuently acts, whit a PWM exit, in the MOSFET that controls the fan but, and the second MOSFET?

- I find this pinout very interesting, so here is the question for the genius(es)...

- Let's say you are stuck in middle of nowhere and you need either AC or heat, is there a ghetto way to get the system working?


1. Jumping the blower fan so the fan works to blow air. This is the easy part.

2. How do you activate AC?

3. How do you activate Heat?
Funny that BMW and Merecedes/Chrysler Crossfire drivers are ALL having issues with their Fan "Regulator" module. I just changed the one on mine due to failure and decided to un-pot the thing and see if I could figure out what went wrong.

I found the same ELMOS IC on mine as is in the BMW units! Also, the same resistor metal strips (Two of them!) and two MOSFETs. YES, they are MOSFETs, here are the specifics and the images from my tear down.

TO-220 package #1 & #2

PN# 2N0609

Idendified as Infineon Part # IPP80N06S2-09

It is a power MOSFET. See attached PDF for details.

SOIC16 wide body package

PN# 10901D

Identified as ELMOS Part #???

Propriatary part. No Manufacturer datasheet available?
From the information I have found on various threads and different forums, the ELMOS IC is a linear voltage regulator. So the "Fan Regulator" part description is at least somewhat accurate.

Both of the MOSFETs in mine were directly on the large heat sink with thermal compound. They would have done better screwing the heat sink tabs to the aluminum heat sink for better thermal conductivity IMHO.

Take a look. Also, here is a video of the part change, you can see the large heat sink used early on in the video.

I know I don't own or drive a BMW, but I figured that MAYBE our two communities could work together to come up with a PWM solution that would allow ALL of us to replace these POS parts :mad: with something that would last :dunno: and get us back to enjoying our vehicles for their primary purpose... the drive! :drive: _sl_


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I found the same ELMOS IC on mine as is in the BMW units! Also, the same resistor metal strips (Two of them!) and two MOSFETs. YES, they are MOSFETs
Very nice. Thank you for posting here.
It certainly looks as if you have the same Germanic Behr garbage we have.
(Do you think the problem is the Elmos 1091d chip? The infineon optimos power transistor ipb80n06s2-09 ipp80n06s2-09) Mosfets? Or the design of the FSU? Or the overall German system design?)

How on earth did you depot yours so nicely? (My depotting endeavor was a disaster!)

MAYBE our two communities could work together to come up with a PWM solution
That would be wonderful!

Is your assessment that the Elmos 10901D went bad?
Or the (infineon optimos power transistor ipb80n06s2-09 ipp80n06s2-09) mosfets?

And, most importantly, with the FSU in our hands, do you think we can come up with a test procedure (e.g., put 6 volts on the center pin, and then measure the output?)?
- How to build a test jig for fsu parasitic battery drain and HVAC anomaly diagnostics (1)

PS: My most recent FSU, put in only a week ago, lasts for 5 minutes, and then shuts off. Every time. I had to return it (see pictures below) and have not received the replacement yet.


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Glad I could help.

Getting that damn thing un-potted was very testing of my patients. VERY small flat head screwdrivers and a X-Acto knife were my friends. I got lucky prying the board out as i had uncovered both the MOSFETs first and cleared the perimeter of the board before attempting to lift it out. Even then I took it VERY slow so as not to break or damage the board.

All the evidence to date suggests that we can put 12VDC on two of the input pins with 0-6 or 0-7 VDC on the 'control' pin with a common ground with the 12VDC. Any 12VDC motor drawing less amperage than our fan motors SHOULD work as a test load.

Granted this would not duplicate the conditions in the vehicle. To do that, you would need to procure a fan from the salvage yard and use that for your test load and a car battery or car charger for your 12VDC source. Unless you have a 30 or 40 amp bench 12VDC supply just laying around (yeah, right!).

I am past the point of trying to repair these POS parts and would like to get a PWM controller built so I never have to bother with this again. What research I have done on the subject states that a 20khz frequency would keep audible noise to a minimum and still allow efficient switching of the power to the motor without all that waste heat that requires such a huge heat sink.
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Getting that damn thing un-potted was very testing
Thanks. Now at least I don't feel so much like a klutz!

we can put 12VDC on two of the input pins with 0-6 or 0-7 VDC on the 'control' pin with a common ground with the 12VDC. Any 12VDC motor drawing less amperage than our fan motors SHOULD work as a test load.
Thanks. I will put this information into the aforementioned test-jig thread.

you would need to procure a fan from the salvage yard and use that for your test load and a car battery
That would work but maybe the test load is even easier.
Do you think we could simply use the old connector from a depotted FSU and connect it to the vehicle, with two wires sticking out that are connected to the blower motor?

I am past the point of trying to repair these POS parts and would like to get a PWM controller built
I would love to see that, but, I myself, don't have the skills.
Maybe the BMW team members who understand this stuff better than I can pitch in on the design work?

BTW, are you sure it's the lousy Germanic FSU design that is the culprit?
Maybe it's poor quality components (Elmos_1091D chip? Mosfets?)?
Or the overall German HVAC system design?

BTW, I don't think we've identified our mosfets yet.
How did you know yours were the Infineon Optimos power transistor ipb80n06s2-09 ipp80n06s2-09 MOSFETS?
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